Supply device for power supply to an electronic unit in a semiconductor valve in a shunt-connected thyristor-switched capacitor

ABSTRACT

A supply device (FD 1 , FD 2 ) for power supply to an electronic unit (EU 1 , EU 2 ) for a controllable semiconductor element (T 1 , T 2 ) in a semiconductor valve in a shunt-connected thyristor-switched capacitor (CA). The capacitor being intended to carry an alternating current with a known period (T), the semiconductor valve comprising a snubber circuit (SC) with a first and a second terminal (CS 1  and CS 2 , respectively) only. The supply device has an energy storage (C 1 , C 2 ) for storing electrical energy, a valve terminal (J 13 , J 23 ), a snubber terminal (J 12 , J 22 ), a supply terminal (J 11 , J 21 ) connected to the energy storage and a first current path from the snubber terminal to the supply terminal. The energy storage is designed to store an amount of energy which is larger than the energy requirement of the electronic unit during one cycle of the alternating current but smaller than its energy requirement during two cycles.

TECHNICAL FIELD

The present invention relates to a supply device for power supply to anelectronic unit for a controllable semiconductor element in asemiconductor valve in a shunt-connected thyristor-switched capacitor,the capacitor being intended to carry an alternating current with aknown period, the semiconductor valve comprising a snubber circuit fortransient protection of the semiconductor element with a first and asecond terminal only.

BACKGROUND ART

It is known to connect, to electric power networks in shunt connection,static compensators for compensation of the reactive power consumptionof the power network and of equipment connected to the power network.One type of such compensators comprises at least one and usually aplurality of thyristor-switched capacitors (TSC). A thyristor-switchedcapacitor substantially comprises a capacitor in series connection witha controllable semiconductor valve. In addition thereto, an inductiveelement, an inductor, is usually arranged in series connection with thecapacitor to limit the rate of change of the current through thecapacitor when the capacitor is connected to the power network and toavoid resonance phenomena with inductive components located in the powernetwork.

The controllable semiconductor valve comprises at least two controllablesemiconductor elements, usually thyristors, arranged in anti-parallelconnection. By bringing the semiconductor elements in a conductingstate, that is, by controlling their firing time relative to the phaseposition of the voltage of the ac network, the capacitor may be coupledto the power network for generating reactive power. It is to beunderstood that, in this application, the concept capacitor comprisesalso those cases where the capacitor is composed of a plurality ofmutually connected capacitive elements, sub-capacitors, which are allcommonly coupled by the controllable semiconductor valve. Further, it isto be understood that the semiconductor valve may comprise a pluralityof mutually series-connected, and then usually pair-wiseantiparallel-connected, semiconductor elements, which are eachcontrolled by a firing order. A control device generates individualfiring pulses for the semiconductor elements included in thesemiconductor valve.

FIG. 1 illustrates a static compensator of the kind described above,which is connected via a transformer TR to an ac network N1. Thecompensator comprises three capacitors CA, CB, CC, each beingshunt-connected to a common voltage busbar BB via a controllablesemiconductor valve VA, VB, VC, respectively, and an inductor LA, LB,LC, respectively. The semiconductor valves are schematically illustratedin the figure with two semiconductor elements T1, T2 in antiparallelconnection. Control equipment CEQ supplies firing orders COA, COB, COC,respectively, to the semiconductor valves.

For a general description of thyristor-switched capacitors and controlthereof, reference is made to, for example, {dot over (A)}ke Ekström:High Power Electronics HVDC and SVC, Stockholm 1990, in particular pages10-1 to 10-7.

Since the current through the thyristor-switched capacitor in steadystate has a phase position 90 electrical degrees in advance of thevoltage across the same, the two antiparallel-connected semiconductorelements of the semiconductor valve should be given firing ordersalternately at the times when the time rate of change of the fundamentaltone for the voltage across the thyristor-switched capacitor changessign from a positive value to a negative value, and inversely. If thephase position of the voltage is defined such that, at 0°, its amplitudeis zero and increasing in a positive direction, under steady-stateconditions these sign reversals take place at the electrical angles 90°and 270°. When the above-mentioned time rate of change changes sign froma positive to a negative value, a firing order should be given to thatof the semiconductor elements, the conducting direction of whichcoincides with the expected current direction in the next interval, thatis, with the above-mentioned convention, in the interval 90° to 270°.When the mentioned time rate of change again changes signs, a firingorder is given to the other semiconductor element, the conductingdirection of which coincides with the expected current direction in theinterval which is then to follow, that is, with the above-mentionedconvention, in the interval 270° to 450°.

When the generation of firing orders is brought to an end, for examplein dependence on a voltage control system for maintaining the voltage inthe ac network or the voltage busbar BB constant, the current throughthe semiconductor valve will cease at the next zero crossing of thecurrent. The voltage of the capacitor thus remains at a level determinedby the voltage of the power network when the current through thecapacitor was forced to cease. When a firing order is again generated,according to the criterion mentioned above, and the voltage of thevoltage busbar has remained unchanged, the connection of the capacitoroccurs, in principle, without any transition phenomena in current andvoltage.

Usually, each semiconductor element is associated with an electronicunit with an indicating device which, in some manner known per se,generates indicating signals, indicating that an off-state voltageexists across the semiconductor elements, in the respective conductingdirection of the semiconductor elements. Typically, an indicating signalis generated when the off-state voltage amounts to about 50 V across asemiconductor element in the form of a thyristor. These indicatingsignals are usually transferred from the potential of the semiconductorsvia light guides to the control equipment arranged at ground potential.

Likewise, in some manner known per se, the control equipment generates,in dependence on received indicating signals, firing orders and supplythese to the electronic units, also usually via light guides. Ingeneral, therefore, the electronic units comprise circuits withcomponents, for example photodiodes, for transforming the firing orderin the form of light into electrical firing signals for each of thesemiconductor elements.

To limit current and voltage stresses on the semiconductor elements inconnection with a change of their conducting state, a transientprotection circuit, a so-called snubber circuit, is usually arranged inparallel connection with the semiconductor elements, this circuitcomprising a series connection of resistive and capacitive components.

The above-mentioned functions of the electronic units require electricalenergy and the electronic units must therefore have access to a powersupply. This power supply should be galvanically separated from groundpotential and the electric power should thus be supplied from that acnetwork to which the thyristor-switched capacitor is connected.

The electronic units usually also comprise a gate circuit whichforwards, to the semiconductor elements, firing orders received from thecontrol equipment for firing the respective semiconductor element independence on the voltage level of the supply voltage.

A known way of arranging this power supply for thyristor-switchedcapacitors is illustrated in FIG. 2. The figure schematicallyillustrates parts of a semiconductor valve of the kind described above,which comprises two thyristors T1, T2 in antiparallel connection, asnubber circuit SC with a snubber capacitor CS and a snubber resistor RSin series connection. Supply devices FD1 and FD2, respectively, areadapted to supply electronic units (not shown in the figure) for thethyristors T1, T2, respectively, with electrical energy. Each one of thesupply devices comprises an energy storage in the form of a capacitor,in the figure designated C1 and C2, respectively. The voltage across thecapacitors, in the figure designated UF1 and UF2, is supplied to therespective electronic units. A current transformer—not shown in itsentirety in the figure—with a primary winding, through which thealternating current through the thyristor-switched capacitor flows, hasa number of separate secondary windings, two of which, designated S1 andS2, respectively, are shown in the figure. The supply device FD1 furthercomprises diodes Da1 and Da2. When current flows through the secondarywinding S1, a current path through the supply device FD1 is closed viathe diode Da2, the capacitor C1, and via a Zener diode Zc in a supplydevice FD2′, which is adapted for power supply of an electronic unit(not shown) for a thyristor T2′, connected in series with the thyristorT2. In the event that the supply device FD2′ does not exist, the currentpath is instead closed via a Zener diode Za′ in the supply device FD1.The capacitor C1 is thus supplied with energy via the current throughthe secondary winding S1.

The thyristor T1 has one anode terminal TA1 and one cathode terminalTC1. When no current flows through the current transformer, that is,when the semiconductor elements are in a non-conducting state, and whenthe voltage between the anode and cathode terminals exhibits a positivetime rate of change, a small amount of energy is supplied to thecapacitor C1 through a current path from the anode terminal TA1 via adiode Db1 in the supply device FD2, the snubber circuit SC and a diodeD11. Conventionally, the energy storage is designed to contain energysufficient for the safe function of the electronic unit for a pluralityof cycles of the alternating current, which, however, also implies thata plurality of ac cycles are required for supplying, via the snubbercircuit, an amount of energy which is large enough for the energystorage to attain a voltage level and an energy content sufficient forthe safe function of the electronic unit. Thus, this solutionpresupposes that the energy requirement of the electronic unit isensured via supply from a current transformer, which component, ofcourse, complicates and renders more expensive the system for energysupply to the electronic units.

FIG. 3 illustrates a known system for energy supply to electronic unitsof a corresponding kind in a semiconductor valve included in a converterfor conversion between alternating current and high-voltage directcurrent. A thyristor T1 included in the semiconductor valve has asnubber circuit SC connected between the anode terminal TA1 and thecathode terminal TC1. In this case, the snubber circuit comprises afirst series connection of a resistor RS1 and a capacitor CS1, which inturn is connected in series with a second series connection of aresistor RS2 and a capacitor CS2. A third series connection of acapacitor CS3 and a resistor RS3 is connected between the point ofconnection between the above-mentioned capacitors and a supply deviceFDH for energy supply of an electronic unit (not shown in the figure)for the thyristor T1. When the voltage between the anode and cathodeterminals exhibits a positive time rate of change, a current path isformed from the anode terminal via the first and third seriesconnections, a diode D11 in the supply device and an energy storage inthe form of a capacitor C1h to the cathode terminal. The voltage acrossthe capacitor, designated UF1h in the figure, is supplied to theelectronic unit. In this case, the energy storage is designed to becharged during each cycle, via the current path mentioned, with anamount of energy which is large enough for the energy storage to attaina voltage level and an energy content sufficient for the safe functionof the energy unit during one cycle of the alternating current. In thiscase, the snubber circuit is designed as a voltage-divider circuit,which implies that only part of the current through the snubber circuitis supplied to the supply device.

SUMMARY OF THE INVENTION

The object of the invention is to achieve an improved supply device orthe kind mentioned in the introductory part of the description, whicheliminates the need of energy supply via a current transformer, utilizesin full the current through the snubber circuit for energy supply to thesupply device and to the electronic unit, and which, through the designof its circuitry, contributes to a simple, reliable and economicallyadvantageous design.

According to the invention, this is achieved by arranging the snubbercircuit of the semiconductor element with only one first and one secondterminal, the supply device with an energy storage for storingelectrical energy, a valve terminal, a snubber terminal, a supplyterminal connected to the energy storage, and with a first current pathfrom the snubber terminal to the supply terminal, the valve terminal forconnection to the cathode terminal, the snubber terminal for connectionto one of the terminals of the snubber circuit, and the supply terminalfor connection to the electronic unit, the snubber circuit forconnection such that a second current path, for carrying a chargecurrent to the energy storage in a direction from the snubber terminalto the cathode terminal, is formed from the anode terminal through thesnubber circuit to the snubber terminal, and therefrom via the energystorage to the cathode terminal, and by designing the energy storage tostore an amount of energy which is larger than the energy requirement ofthe electronic unit during one cycle of the alternating current butsmaller than its energy requirement during two cycles.

In this way, the current transformer may be eliminated and a currentpath may be formed for carrying the whole current through the snubbercircuit direct to the electronic unit and to the energy storage of thesupply device.

Advantageous improvements of the invention will become clear from thefollowing description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail by description ofembodiments with reference to the accompanying drawings, wherein

FIG. 1 shows, in the form of a single-line diagram, a static compensatorwith thyristor-switched capacitors,

FIG. 2 shows a known embodiment of a power supply of electronic unitsfor thyristors in a thyristor-switched capacitor according to FIG. 1,

FIG. 3 shows a known embodiment of a power supply of electronic unitsfor thyristors in a semiconductor valve in a converter for conversionbetween alternating current and high-voltage direct current, and

FIG. 4 shows an embodiment according to the invention of a power supplyof electronic units for thyristors in a thyristor-switched capacitoraccording to FIG. 1,

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 shows an embodiment of supply equipment according to theinvention. A semiconductor valve comprises two semiconductor elements inthe form of thyristors T1, T2 in antiparallel connection, each one withan electronic unit EU1, EU2, respectively. The electronic units are ofthe kind described above and form indicating signals IP1, IP2 which aresupplied to control equipment CEQ. The control equipment forms firingorders FO1, FO2, as described above, and supply these to the electronicunits. The signal transmission takes place via light guides LIC1, LIC2.Each of the thyristors has an anode terminal TA1, TA2, respectively, anda cathode terminal TC1, TC2, respectively. The anode terminal TA1 andthe cathode terminal TC2 are connected to an inductor LA and the anodeterminal TA2 and the cathode terminal TC1 are connected to a capacitorCA. The inductor, in its turn, is connected to an ac circuit (not shownin the figure) with a known period T, for example as illustrated in FIG.1.

Further, the semiconductor valve comprises a snubber circuit SC with aseries connection of a resistor RS and a capacitor CS and with only afirst terminal CS1 and a second terminal CS2.

A first supply device FD1 for power supply to the electronic unit EU1comprises an energy storage in the form of a capacitor C1 for storingelectrical energy, a valve terminal J13, a snubber terminal J12, and asupply terminal J11. The capacitor C1 is connected between the valveterminal and the supply terminal. The valve terminal is intended forconnection to the cathode terminal of the semiconductor element, thesnubber terminal for connection to one of the terminals of the snubbercircuit, in this case to the terminal CS1, and the supply terminal forconnection to the electronic unit. The supply device comprises a firstcurrent path from the snubber terminal via a resistor R1 and a diode D11to the supply terminal such that a current may flow from the snubberterminal to the supply terminal but not in the reverse direction. Forpower supply to the electronic unit EU2, a second supply device FD2 ofthe same kind is provided, the components included in the supply devicesand the terminals of the same kind being designated in the figure withthe corresponding designations, whereby, for the second supply device,FIG. 1 in the first figure of the reference numerals is replaced by FIG.2.

The snubber circuit is connected such that a second current path, forcarrying a charge current to the energy storage in a direction from thesnubber terminal to the valve terminal, is formed from the anodeterminal through the snubber circuit to the snubber terminal, and fromthis via the energy storage and the valve terminal to the cathodeterminal. The supply devices further comprise a third current path,parallel with the second current path, from the respective valveterminals via diodes D12 and D22, respectively, to the respectivesnubber terminals, for carrying a parallel current in a direction fromthe valve terminal to the snubber terminal and further to the snubbercircuit, but not in the reverse direction. The snubber circuit is thusconnected as a series circuit between the snubber terminal J12 in thefirst supply device FD1 and the snubber terminal J22 in the secondsupply device FD2.

A breakdown diode in the form of a Zener diode Z11 is connected betweenthe supply terminal and the valve terminal for limitation of the voltageUF1 of the supply terminal.

The first current path comprises a branch point, which in thisembodiment of the invention consists of the supply terminal J11, suchthat the first and second current paths coincide with each other betweenthe snubber terminal and the branch point mentioned. The resistor R1,which constitutes a current-limiting element, and the diode D11, arearranged in the coinciding part of the two current paths.

When, for example, the thyristor T1 is to assume the current through thecapacitor CA, the condition for firing of the same is that the off-statevoltage in its forward direction exceeds a predetermined level, which istransferred to the control equipment CEQ from the electronic unit EU1via an indicating signal. In dependence on known quantities in theinstallation, the control equipment generates, in some manner known perse, firing orders FO1, which are supplied to the electronic unit when acorresponding indicating signal has been received.

The build-up of the off-state voltage across the thyristor T1 impliesthat the time rate of change of the voltage between the anode terminaland the cathode terminal will have a positive sign, that the thirdcurrent path in the second supply device FD2 is brought to a conductingstate and that a charge current will flow through the snubber circuit SCin a direction from the terminal CS2 to the snubber terminal J12 in thefirst supply device. The third current path in the first supply deviceis in a nonconducting state, so the charge current in the first supplydevice flows through the coinciding part of the first and second currentpaths and, at the branch point between them, is divided into a currentwhich flows directly to the electronic unit and a current which chargesthe capacitor C1. As long as the voltage UF1 at the supply terminal J11,that is, across the capacitor C1, is below the breakdown voltage of theZener diode Z11, the capacitor is charged with the charge current. Whenthe mentioned voltage reaches the breakdown voltage, the charge currentwill flow through the Zener diode.

When the thyristor T1 fires, the voltage across the same returns to avalue near zero, whereby a current of short duration flows through thesnubber circuit in a direction from the cathode terminal via the diodeD12, via the snubber circuit, and in the second supply device FD2 viathe resistor R2 and the diode D21 to the capacitor C2 and hence providesa charge addition thereto.

The function in case the thyristor T2 is to assume the current throughthe capacitor CA is completely analogous to the one described above,whereby, in the description, the second supply changes places with thefirst one.

In an advantageous improvement of the invention, a shunt regulator,comprising a controllable switching member in the form of an auxiliarythyristor TX1, is arranged between the snubber terminal and the valveterminal. By means of a breakdown diode, in this embodiment a Zenerdiode Z12, connected between the snubber terminal and the gate of theauxiliary thyristor, the voltage US1 on the snubber terminal is sensed,which voltage thus constitutes a comparison voltage for thepredetermined breakdown voltage UZ12 of the Zener diode. When thecomparison voltage exceeds the breakdown voltage, the auxiliarythyristor is brought to a conducting state by the current through thebreakdown diode and then closes a fourth current path from the snubberterminal via the auxiliary thyristor to the valve terminal for carryingthe charge current past the capacitor C1. The voltage at the snubberterminal depends on the sum of the voltage at the supply terminal andthe voltage across the resistor R1, whereby the shunt regulatorintervenes in dependence on the voltage at the supply terminal as wellas the amplitude of the current in the coinciding part of the first andsecond current paths.

Typical component values for a supply device according to the inventionare C1=1 μF, R1=1 Ω, RS=10 Ω and CS=2 μF. The breakdown level of theZener diode Z11 is typically 25 V and of the Zener diode Z12 typically47 V. In the known embodiment of a supply device for the same purpose,described with reference to FIG. 2, the capacitor C1 is usually designedto have a capacitance value of typically 20 to 30 μF, that is, typicallyof an order of magnitude greater than in the embodiment according to theinvention.

When designing the supply device according to the invention, which isdone with knowledge of the energy and voltage requirements of theelectronic unit, the energy storage, that is the capacitor C1, isdesigned to store an amount of energy which is larger than the energyrequirement of the electronic unit during one cycle of the alternatingcurrent but smaller than its energy requirement during two cycles.

This implies that the energy storage is practically emptied inconnection with the thyristor T1 being brought to a conducting state. Atthe next current pulse through the snubber circuit, part of this currentpulse may be passed directly to the electronic unit via the firstcurrent path. By this series connection of the snubber circuit and thementioned design of the energy storage, a simple and inexpensive designof the supply equipment, which utilizes in full the current through thesnubber circuit, is obtained.

It is to be understood that, in those cases where the semiconductorvalve comprises a plurality of mutually series-connected thyristors,each of these is equipped with a supply device of the kind describedabove and connected over thyristors, which are antiparallel-connected inpairs, in the way illustrated in FIG. 4.

The thyristor-connected capacitor may, of course, also be connected, forexample, between two phases in a three-phase ac network.

What is claimed is:
 1. A supply device (FD1, FD2) for power supply to an electronic unit (EU1, EU2) for a controllable semiconductor element (T1, T2) in a semiconductor valve in a shunt-connected thyristor-switched capacitor (CA), the capacitor being intended to carry an alternating current with a known period (T), the semiconductor valve comprising a snubber circuit (SC) with a first and a second terminal (CS1 and CS2, respectively) only, the semiconductor element with an anode terminal (TA1, TA2) and a cathode terminal (TC1, TC2), the supply device with an energy storage (C1, C2) for storing electrical energy, with a valve terminal (J13, J23), a snubber terminal (J12, J22), a supply terminal (J11, J21) connected to the energy storage, and with a first current path from the snubber terminal to the supply terminal, the valve terminal for connection to the cathode terminal, the snubber terminal for connection to one of the terminals of the snubber circuit, and the supply terminal for connection to the electronic unit, the snubber circuit for connection such that a second current path, for carrying a charge current to the energy storage in a direction from the snubber terminal to the cathode terminal, is formed from the anode terminal through the snubber circuit to the snubber terminal, and therefrom via the energy storage and the valve terminal to the cathode terminal, characterized in that the energy storage is designed to store an amount of energy which is larger than the energy requirement of the electronic unit during one cycle of the alternating current but smaller than its energy requirement during two cycles.
 2. A supply device according to claim 1, characterized in that it comprises a third current path parallel to the second current path from the valve terminal to the snubber terminal for carrying a parallel current in a direction from the valve terminal to the snubber terminal and further to the snubber circuit.
 3. A supply device according to claim 1, characterized in that it comprises a shunt regulator (Z12, TX1 and Z22, TX2, respectively) which, when a comparison voltage (US1, US2), depending on the voltage level of the energy storage, exceeds a predetermined voltage level, closes a fourth current path for the charge current past the energy storage.
 4. A supply device according to claim 1, characterized in that the first current path comprises a branch point (J11, J21), that the first and second current paths coincide with each other between the snubber terminal and said branch point, and that the coinciding part of the current paths comprises a current-limiting element (R1, R2).
 5. A supply device according to claim 4, characterized in that said comparison voltage is dependent on a sum of the voltage (UF1, UF2) at the supply terminal and the voltage across said current-limiting element.
 6. A supply device according to claims 3, characterized in that said shunt regulator comprises a breakdown diode (Z12, Z22), for example a Zener diode, for sensing the comparison voltage, and a controllable switching member (TX1, TX2) which closes said fourth current path in dependence on a current through said breakdown diode. 